The first example comes in the caption of the comic "Cornered," where the mom is saying to the kid(?): "Kindergarten is supposed to be fun. You mustn't be doing it right." I would have said* "You must not be doing it right"; I hear "you mustn't" as meaning "you are forbidden to," not "you probably are." (I wondered if the "mustn't" form might be OK in British English, but I thought Americans would be on my side here.)
The tagline in a Slim-Fast ad bugged me in a slightly different way. "Your swimsuit is ready. You'll be too" trips me up; I'd prefer either "You'll be ready too" or "You will be too."
I hoped these contractions would prove to be forbidden forms of auxiliary reduction, but just coming up with that term stretched my expertise to its limits. So I asked Arnold Zwicky, who knows all the linguistic labels and is a wizard at coining new ones when they're called for. And he poured cold water all over my quest.
Are these contractions off limits, or even suboptimal? He didn't think so. "For me, the Cornered example is perfectly fine," he wrote.
"Mustn't" (like "must not") is in principle capable of either a root interpretation -- subject is obliged not to V -- or an epistemic interpretation -- it must not / mustn't be the case that subject Vs. Compare: You mustn't touch the hot stove (or you'll get burned). You mustn't like ice cream (if you make a face like that).
As for my theory that the "epistemic mustn't" is a British thing, well, that's just an example of the "foreigners are weird" default assumption. Americans, Brits, and Australians all use it, said Zwicky. It's true that "some speakers judge such examples to be unacceptable, but what they say about them is all over the map. In particular, some find the examples to be unacceptably American, some find them to be way formal and British-sounding."
The swimsuit issue was also not a question of auxiliary reduction, he thought.
I think that the problem here is that the example is somewhat zeugmatic: in "Your swimsuit is ready", the subject of "be ready" is interpreted as the object of the understood verb "wear", but in "You'll be ready", the subject of "be ready" is interpreted as the subject of "wear". These are non-parallel interpretations. (Things are fine in "I'll be ready, and you'll be too.")Well, maybe. But I don't interpret the elliptical sentence as "Your swimsuit is ready [to wear], and you'll be [ready to wear it] too." My (mental) expansion was "Your swimsuit is ready [for summer, for the beach], and you'll be [ready for summer] too" -- which is grammatically parallel. It's the truncation itself that doesn't work: For me, things are not fine in "I'll be ready, and you'll be too," any more than they're fine in "I'm happy, and you're too." (You're too what?)
That's as far as I can go without professional help. And if you want to avoid this whole mess, I understand. But if you've been patient enough to read this far, please let me know how these constructions strike your ear -- intolerable, unremarkable, or somewhere in between?
*At least I think I would -- we're all unreliable witnesses to our own linguistic behavior.